Refer to figures.
An decrease in atmospheric pressure means (distance between air molecules getting larger) that we have “less denser” air to use.

Less denser air used by engines so the performance will be worse if we compare with use more denser air.
It effects negative for landing (increase) distance (less drag) and go around (degraded) performance (less thrust).

Less air mass flowing over your wings prevents you from generate as much lift, and less oxygen mass in your cylinders prevents you from burning as much fuel, meaning less power. Decreasing air density decreases performance, so be careful on hot days at high altitudes.